1. Field of the Invention
The present invention relates to a fuel-vapor treatment method and apparatus for an internal combustion engine, wherein, to prevent air pollution and fuel loss, fuel vapor (hereinafter called the vapor) emitted from a fuel tank is temporarily stored and then released into an intake system in accordance with the engine operating condition.
2. Description of the Related Art
Generally, in prior known fuel-vapor treatment methods and apparatus, fuel vapor is adsorbed onto an adsorbent, such as activated charcoal, contained in a canister, and during engine operation, the adsorbed vapor is desorbed and released into an intake system by utilizing the negative pressure created by engine intake stroke (this process is hereinafter called the "canister purge"). Control of the purge is performed by using a solenoid valve installed in a purge passage communicating between the canister and an intake passage downstream of the throttle valve, the solenoid valve being controlled to open or close the purge passage depending on the engine operating condition.
More specifically, when the engine operating condition has passed from an operating range where purge is not performed (low load, low rpm range) to an operating range where purge is performed (high load, high rpm range), the solenoid valve is opened. Conversely, when the engine operating condition has passed from the operating range where purge is performed to the operating range where purge is not performed, the solenoid valve is closed. During purging, the duty cycle of the solenoid valve is controlled to control the purge gas amount in accordance with the engine operating condition. In a commonly employed method for such duty cycle control, control is performed so that a prescribed purge gas amount is obtained proportional to the intake air amount, that is, the purge ratio (the ratio of the purge gas amount to the intake air amount) is maintained constant. For the above prior art, refer, for example, to Japanese Patent Unexamined Publication No. 4-72453 (corresponding U.S. Pat. No. 5,323,751).
However, in the intake-air-amount-proportional purge as described above, no serious consideration has been given to the density of vapor to be purged into the intake passage. That is, the vapor actually charged into the intake passage through the solenoid valve includes not only the vapor from the canister but also the fuel vapor being emitted directly from the fuel tank. The density of the vapor purged into the intake passage is therefore determined by the tank vapor, the canister vapor, and the purge air amount which is the amount of atmosphere drawn into the canister. While the canister vapor amount increases in proportion to the purge air amount, the tank vapor amount tends to be held substantially constant regardless of the purge air amount.
Accordingly, when the density of the canister vapor is low and the density of the tank vapor is high, if the solenoid valve is operated to vary the purge gas amount, the density of the resulting mixture will vary greatly. More specifically, when the solenoid valve is operated toward an open position to increase the purge gas amount, only the canister vapor amount increases while the tank vapor amount remains constant. As a result, the overall density of the vapor mixture charged into the intake passage through the solenoid valve decreases because of the low density of the canister vapor; therefore, in a control operation for correcting the fuel injection amount based on the purge gas amount, the moment that the solenoid valve is operated toward its open position, the air-fuel mixture becomes lean, hence causing perturbations in the air-fuel ratio. On the other hand, when the solenoid valve is operated toward a closed position to decrease the purge gas amount, only the canister vapor amount decreases while the tank vapor amount remains constant. As a result, the overall density of the vapor mixture charged into the intake passage through the solenoid valve increases since the density of the canister vapor is low; therefore, in the control operation for correcting the fuel injection amount based on the purge gas amount, the moment that the solenoid valve is operated toward its closed position, the air-fuel mixture becomes rich, hence causing perturbations in the air-fuel ratio.